What size guide scope?

I plan on using a Meade Pictor 201 XT autoguider with my Meade 127 ED and am wondering what size guide scope to use? 50mm, 60mm, 80mm? I know the 201 is mag 11 limited and so is a 80mm scope, but I am not sure about the size and even the focal ratio, f/6 or f/11?
The camera used will be a Meade DSI II Color one shot.

Floyd,
I'm no expert but it would seem to me that regardless of what size scope you choose you would want to end up guiding near or above the resolution that you are going to be shooting at.

After matching the focal length to to the camera you could pick the aperature based on what your mount can handle.

The 201 is difficult at best to guide with. I've owned one. Get a used SBIG ST5 or 237 with their Efinder. Very small light weight guider that is good for imaging up to 2000mm.

If you're a very patient man you may get some good results. For me, thumbing through the manual trying to figure out what all the numbers mean and getting the timing right for the "short, medium and long" presses
was a bit of a challenge.
A couple of things that may help your success in case you didn't know.
1. Whatever guide scope you use, a 9mm eyepiece is very close to being parfocal with the pictor.
2. Be sure you enable the "SC" for Seeing Conditions. Other wise the pictor will try to case star movement caused by turbulent skies.
Good luck and I look forward to seeing some images.

A recent thread regarding this topic was discussed on the SBIG Yahoo!Groups. Much discussion ensued, some even with math formulas! The consensus was that if you use a guidescope then you should try to use a guide focal length of 1/3rd to 1/6th of your imaging focal length. I currently guide at f.l. 320mm, yet image at around 1100mm. When I go to 2500+mm imaging f.l. I'm going to an OAG (to take flexure out of the picture).

All that said, if you have a good solid mount, good polar alignment, low PEC (use PEMPro), and adjust your guider min and max moves with CCDWare guider calculator then you will likely do just fine with any number of setups.

Best,

Tom